BOND-COUNTING APPROACH FOR REPRESENTING ASSOCIATION EFFECTS IN THE INTERFACIAL REGION OF MULTICOMPONENT SYSTEMS

Explicit expressions for the excess Helmholtz free energy due to hydro gen bonding in multicomponent systems, containing either a solid-liqui d or a vapor-liquid interface, have been derived. The approach followe d in the present article is based on the works of Levine and Perram (1 968), who proposed that the. focus should be on the correct counting o f H-bonds in the system. This approach is distinctly simpler than the reaction equilibrium theory (RET) or the thermodynamic perturbation th eory (TPT), proposed earlier by Suresh and Naik (1996), which focuses on the distribution of chain length of H-bonded oligomers/clusters. In this letter, we demonstrate that, despite the striking difference in the conceptual physical picture of fluids as visualized by RET/TPT and the bond-counting approach, the expressions for the excess free energ y of the system in the interfacial region due to H-bonding, derived us ing either approaches, turn out to be entirely identical. While the ma thematical and physical origins of this coincidence need to be investi gated, this finding opens up an opportunity to rigorously and quantita tively incorporate association effects into rigorous, but mathematical ly complex, multilayer theories for interfacial phenomena in a straigh tforward and simple manner.